Mother's Day x Labour Day

Estudando um pouco me deparei com as seguintes estruturas: "sem o uso do genitive"
Labour Day
Indian Day
Flag Day
Com o uso de genitive:
Mother´s Day
Valentine´s Day
Children´s Day
Qual é o critério do "uso" e "não uso" do genitive nesses casos ?
Thanks a lot !
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Oh! A good question! I learned from this one, really!
In the case of Mother´s day many people advocate that it can be both ways, because if you think of a day that is "of" your mother, not a group of mothers, not all mothers of the world. But, of course there are others that think otherwise. So, it depends on which camp you side with.
Anyway, the most credible answer I got is here:
http://www.englishdaily626.com/qna.php?015
Labour Day - "Labor" refers to the work done as well as the workers collectively. We don't normally use an apostrophe 's' after the word "labor" to indicate possession. (because nobody "owns" the labor).

Mother´s Day - you celebrate a person, it´s less abstract notion, it´s your mother that you celebrate.
"Mother" and "father' are people and the days chosen to honor them are called their days.
In a way, those are supposed to be about living people, somehow. The same goes to the Children´s day case. Whereas Indian Day, Labour Day, etc are of a more distant past. And not taken collectively/thought collectively, sort of.

The following links I had seen before the above one, the subject makes a good trivia discussion!
http://metro.co.uk/2014/03/30/mothers-d ... s-4680159/
http://www.betterwritingskills.com/tip-w011.html